In a manufacturing process of a semiconductor device, as a plasma processing apparatus for processing a processing target object such as a semiconductor wafer, there is known a plasma etching apparatus which is configured to perform an etching process on the processing target object by irradiating plasma to the processing target object. For example, a capacitively coupled parallel plate type plasma etching apparatus is widely utilized as the plasma etching apparatus.
In the capacitively coupled parallel plate type plasma etching apparatus, a pair of parallel plate electrodes (i.e., an upper electrode and a lower electrode) is provided within a processing vessel, for example. The processing target object is placed on the lower electrode. By supplying a processing gas into the processing vessel and applying a high frequency power to the upper electrode or the lower electrode, plasma is generated within a space between the upper electrode and the lower electrode, and etching is performed on the processing target object placed on the lower electrode with the generated plasma.
Recently, in the manufacturing process of the semiconductor device, it is required to form a hole having a higher aspect ratio by processing the processing target object. As commonly known in the art, however, if the aspect ratio is increased, positive ions are collected in a bottom portion of the hole, and straightness of the positive ions within the hole is deteriorated. As a result, it is difficult to obtain a desirable etching shape.
As a resolution to this problem, in order to electrically neutralize the positive ions collected in the bottom portion of the hole, there is proposed a method of applying a negative DC voltage to the upper electrode to accelerate electrons emitted from the upper electrode toward the processing target object and supply the accelerated electrons into the bottom portion of the hole.
The neutralization of the positive ions by the supply of the electrons, however, may be hindered by a plasma sheath generated on the processing target object. That is, since the supply of the high frequency power to the lower electrode accompanies generation of the plasma sheath on the processing target object, the electrons may be bounced off the plasma sheath and may not reach the bottom portion of the hole. As a consequence, the positive ions may not be sufficiently neutralized by the supply of the electrons.
Accordingly, there is proposed a plasma etching apparatus in which a supply amount of electrons is further increased. In this plasma etching apparatus, by turning on and off a high frequency power supply on a preset cycle, a supply of a high frequency power for plasma generation to the lower electrode and a stop of the supply of this high frequency power are alternately repeated. Further, in this plasma etching apparatus, a negative DC voltage having a relatively small absolute value is supplied to the upper electrode for a time period during which the high frequency power is being supplied, and a negative DC voltage having a relatively large absolute value is supplied to the upper electrode for a time period during which the supply of the high frequency power is stopped. Here, for the time period during which the supply of the high frequency power is stopped, the plasma extinguishes, and, thus, the plasma sheath on the processing target object is reduced or extinguished. In this time period, since the negative DC voltage having the relatively large absolute value is supplied to the upper electrode, a larger number of electrons is supplied into the bottom portion of the hole in the state that the plasma sheath on the processing target object is reduced or extinguished. Accordingly, the positive ions collected in the bottom portion of the hole can be efficiently neutralized.
Patent Document 1: Japanese Patent Laid-open Publication No. 2010-219491
In the prior art, however, it is not considered to suppress an electric discharge which may occur at the upper electrode side due to the bouncing of the electrons off the plasma sheath which is generated on the plasma processing object when the high frequency power is supplied to the lower electrode.
That is, in the prior art, for the time period during which the supply of the high frequency power is stopped, the negative DC voltage having the relatively large absolute value is supplied to the upper electrode, and if the supply of the high frequency power is resumed, the negative DC voltage having the relatively small absolute value is supplied to the upper electrode concurrently with the beginning of the re-supply of the high frequency power. Accordingly, in the prior art, in the state that the plasma sheath generated on the processing target object, which is accompanied by the supply of the high frequency power to the lower electrode, is not fully grown, the electrons emitted from the upper electrode are accelerated toward the processing target object by supplying the negative DC voltage to the upper electrode. Accordingly, the accelerated electrons are bounced off in a direction approaching the upper electrode by the plasma sheath which is being grown in a direction away from the processing target object, i.e., in the direction approaching the upper electrode. As a result, in the prior art, the electric discharge may occur at the upper electrode side due to the bounced electrons.